Surge arrestors are devices that are designed to be connected between an electricity line, in particular a medium or high tension line, and ground, for the purpose of limiting the amplitude and the duration of surges that appear on the line.
These surges may be due, for example, to atmospheric phenomena, such as lightning, or to induction in the conductors.
Such surges may also be due to switching operations performed on a line while under tension.
Surge arrestors are generally in the form of a stack of various varistors, and nowadays generally a stack of several disks based on zinc oxide which has electrical resistivity that is highly non-linear as a function of the applied voltage.
More precisely, such varistors allow practically no current to pass so long as the voltage across their terminals remains below a trigger threshold, and in contrast, they pass a very large current that may reach several tens of kA, when the voltage applied across their terminals exceeds the above-mentioned trigger threshold.
The number of varistors used in a surge arrestor is chosen so that the nominal operating voltage on an electricity line is less than the trigger threshold across the terminals of the stack of varistors.
Thus, the surge arrestor can continuously withstand the nominal operating voltage without any current leaking away, while also serving to carry very high discharge currents that may appear temporarily on a line in the event of an accidental voltage surge.
Numerous types of surge arrestor have already been proposed.
The field of surge arrestors has given rise to a literature that is very abundant.
Presently known surge arrestors generally comprise:
a stack of varistors; PA1 two contact pieces of electrically conductive material placed at respective ends of the stack of varistors; and PA1 an envelope of electrically insulative material surrounding the stack of varistors. PA1 i) stacking varistors; PA1 ii) forming a first envelope of composite material on the stack of varistors; and PA1 iii) placing an outer envelope having fins on the first envelope made of composite material; PA1 the method being characterized by the facts that: PA1 step ii) consists in making a first envelope that is at least semi-rigid and that has constant outside section over its entire length, serving in particular to compensate for surface non-uniformities in the stack of varistors due to alignment errors and to dispersions in the dimensions of the varistors; and PA1 step iii) consists in: PA1 a stack of varistors; PA1 a first envelope of composite material formed on the stack of varistors; and PA1 a finned outer envelope placed on the first envelope made of composite material. PA1 the first envelope is at least semi-rigid and is constant in outside section over its length, serving in particular to compensate for surface non-uniformities in the stack of varistors due to defects of alignment and to dispersions in the dimensions of the varistors; and PA1 the finned outer envelope comprises firstly an outer envelope of substantially constant thickness formed by extrusion on the first envelope, and secondly annular fins subsequently installed on the extruded outer envelope.
The above-mentioned envelope of electrically insulative material is itself the subject of literature that is very abundant.
Document GB-A-2 073 965, for example, proposes making said envelope out of a heat-shrinkable material.
Documents U.S. Pat. No. 4,298,900, DE-A-3 001 943, and DE-A-3 002 014 propose additionally installing a porcelain outer housing over the heat-shrinkable envelope.
Documents U.S. Pat. No. 4,092,694 and U.S. Pat. No. 4,100,588 propose placing each varistor in a ring based on silicone and of disposing the stack of varistors surrounded in this way in a porcelain housing.
Document U.S. Pat. No. 2,050,334 proposes placing a stack of varistors in a porcelain housing and of filling the space between the porcelain housing and the stack of varistors with a filler material that is formed, for example, of a halogen-containing compound based on wax.
Documents EP-A-0 008 181, EP-A-0 274 674, EP-A-0 281 945, and U.S. Pat. No. 4,456,942 propose making an envelope surrounding the varistors by means of an elastomer material, in particular formed by being overmolded on the varistors.
More precisely, document EP-A-0 274 674 proposes overmolding an envelope made of a composite material based on elastomer, EPDM, silicone, or any other filled or unfilled resin on a stack of varistors.
Document U.S. Pat. No. 4,161,012 also proposes placing an elastomer envelope on varistors. That document proposes making the envelope by depositing the elastomer on the outside surfaces of the varistors or by molding the envelope onto the varistors, or else by preforming an envelope out of elastomer and then inserting the varistors therein.
As early as 1958, document U.S. Pat. No. 3,018,406 proposed making the envelope in the form of two complementary preformed shells together with an outer envelope of a plastics material that is injection-molded over the varistors.
Document U.S. Pat. No. 3,586,934 proposes making the envelope by means of a synthetic resin, e.g. based on an epoxy or a polyester resin, or else on a polyester or silicone varnish.
Document EP-A-0 196 370 proposes making the envelope on a body of varistors by casting a synthetic resin, formed by epoxy resin, polymer concrete, silicone resin, or an elastomer, or by covering the body of varistors in a tube of heat-shrinkable plastics material, or else by providing said stack with a layer of synthetic resin.
Furthermore, the following documents: U.S. Pat. Nos. 4,656,555, 4,905,118, 4,404,614, EP-A-0 304 690, EP-A-0 335 479, EP-A-0 335 480, EP-A-0 397 163, EP-A-0 233 022, EP-A-0 443 286, and DE-A-0 898 603 propose making the envelope surrounding the stack of varistors out of composite materials comprising fibers, generally glass fibers, impregnated in resin.
More particularly, document U.S. Pat. No. 4,656,555 proposes initially forming a winding of fibers based on a plastics material such as polyethylene or on glass, or on a ceramic, which fibers are optionally impregnated in resin, e.g. epoxy resin, and then forming a housing on the outside of said winding, the housing being made of a polymer material that withstands bad weather, e.g. a material based on elastomer polymers, on synthetic rubber, on thermoplastic elastomers, or on EPDM.
More precisely, that document proposes either preforming the weather-resistant polymer housing, and then engaging the stack of varistors fitted with its fiber winding in said housing, or else initially forming the winding of fibers on the stack of varistors, and then making the housing of weather-resistant polymer material by molding directly on the winding, by spraying the polymer on the winding, or by inserting the stack of varistors together with its winding in a bath of polymer.
Document U.S. Pat. No. 4,404,614 proposes placing successively on a stack of varistors a first envelope based on glass fibers impregnated with resin, e.g. epoxy resin, then a second envelope based on glass flakes and on epoxy resin, and finally a resilient outer envelope based on EPDM rubber or on butyl rubber.
That document specifies that the first envelope, the second envelope, and the outer envelope may be put into place successively on the stack of varistors, or else the envelopes may be formed in the opposite order.
That document also mentions the possibility of molding the outer envelope on the second envelope based on glass flakes and on epoxy resin.
Document EP-A-0 233 022 proposes forming a shell on a stack of varistors, the shell being based on glass fibers reinforced by epoxy resin, and then in placing on said shell an envelope that is based on elastomer and that can be heat-shrunk or that can be released thereon by equivalent mechanical means.
In a variant, the envelope may be molded in situ using a synthetic resin or a polymer material.
That document specifies that the shell may be preformed. That document also proposes using a sheet of preimpregnated fibers.
Document EP-A-0 304 690 proposes initially forming a filamentary winding of resin-impregnated glass fibers, and then in forming a coating of EPDM type elastomer material on the outside of the winding by injection.
Document EP-A-0 355 479 proposes placing the following in succession on the stack of varistors: firstly a barrier formed by a plastics film, e.g. based on propylene; then a winding of non-conductive filaments; and finally a weather-resistant elastomer housing.
Document EP-A-0 397 163 proposes placing the following in succession on the stack of varistors: a winding of filaments impregnated with resin; and then forming a coating having fins on said winding, the coating being made of elastomer, e.g. EPDM, and being formed by injection.
The technique of using a composite material is very old.
As early as 1946, document DE-A-0 898 603 had proposed using resin-impregnated glass fibers for enveloping varistors.
The surge arrestors proposed heretofore have given good service.
Nevertheless, the Applicant seeks to improve existing surge arrestors.